Frequently, signals with a large dynamic range need to be evaluated. Such a signal may, for example, be provided by a variable reluctance sensor. A variable reluctance sensor may be used to measure a speed of rotation, for example of an automotive crankshaft, camshaft, transmission shaft, etc. Speeds of rotation in the range from 0 rpm to 10,000 rpm may need to be measured. Teeth of a rotating toothed wheel may change a magnetic flux through a ferromagnetic pole when passing by the ferromagnetic pole. The magnetic flux increases when a tooth approaches the ferromagnetic pole and decreases when a tooth leaves the ferromagnetic pole. A coil arranged around the ferromagnetic pole may be used to detect a change in magnetic flux. The teeth of the rotating toothed wheel may induce an alternating voltage in the coil. The amplitude and the frequency of the induced voltage may depend on the speed of rotation. Increasing the speed of rotation may increase both the amplitude and the frequency of the induced voltage. The amplitude of the induced voltage may range from about 10 mVpp at low speeds of rotation to about 400 Vpp at high speeds of rotation.
The amplitude of the induced voltage may be clipped or, in other words, be sliced or clamped at a protection voltage. The clipped voltage may have the same shape as the induced voltage for induced voltages smaller in magnitude than the protection voltage. The amplitude of the clipped voltage will not exceed the protection voltage. Therefore, if the protection voltage is chosen small enough, the clipped voltage will not damage electronic devices by exceed a maximum allowable voltage of the electronic device. Further, the reduced voltage range may easily be measured and processed.
However, clipping the induced voltage may degenerate the signal-to-noise ratio of the signal. This may especially be true for noise caused by mechanical irregularities of the toothed wheel, for example, noise caused by bent teeth or leftovers in the gaps between the teeth after punching the teeth, since they have the same frequency as the desired signal. Further, information about the speed of rotation that may be derived from the amplitude may be lost.